After World War I, the Missouri Pacific ordered a 4-8-2 that would be faster than the MT-63, but had to be limited to an adhesive weight of 226,000 pounds. The goal was to use the heating surface as best as possible, what was accomplished by curved circulating plates and thermic syphons. They also got a large superheater with a surface of more than 1,000 square feet. With a driver diameter of 73 inches, it was designated MT-73. Numbers 5308 to 5312 were built by ALCO-Schenectady in 1921, followed by 5313 to 5316 in 1923.

Since these were still not powerful enough for some passenger services, the following two batches were heavier, had a larger boiler, a pressure of 250 instead of 210 psi and roller bearings. These were 5335 to 5339 built by ALCO-Brooks in 1927 and 5340 to 5344 built by ALCO-Schenectady in 1930. In the result these were considerably more powerful, but could only be used on lines which had been upgraded for the higher weight. While the first two batches were retired in 1955, the last two already followed in the next year.

While the first 4-6-2 locomotives still had a narrow firebox and did not bring much advantage over the 4-6-0, the later, classic Pacifics were characterized by a wide firebox above the trailing axle. This allowed a sufficiently large grate area to maintain high speeds with heavy trains, as had been the case with the Atlantic-type express locomotives a few years earlier. While the Pacific was used almost exclusively as express locomotive in Europe, there were also many locomotives in North America with slightly smaller drivers, which were primarily intended for the heaviest, more medium-speed passenger trains.

The first model of this type were the 21 Missouri Pacific locomotives, which were built by ALCO-Brooks in 1902 and formed the basis for the name “Pacific”. They were lighter than other Pacifics which were built in the following years and had a driver diameter of only 69 inches. Together with the abbreviation "P" for “Pacific”, this led to the class designation P-69.

Since the locomotives were partly built directly for the Missouri Pacific and partly for their subsidiary St. Louis, Iron Mountain and Southern Railway, they were given separate number ranges between 1115 and 1123 and between 1616 and 1623. The Stephenson valve gear was later replaced on 20 locomotives by a Baker valve gear and on one locomotive by the Walschaerts valve gear. In 1916 they were superheated, where the cylinder diameter was increased by one inch. The last ones served in this form until 1953.

In the Missouri Pacific class scheme, the designation P-73 stood for 4-6-2 express locomotives with a driver diameter of 73 inches or 1,854 mm. The class consisted of several batches built by two ALCO works between 1910 and 1926, of which the first were built with drivers of 69 inches and later rebuilt to 73 inches. The first 4-6-2 locomotives ordered by this railroad, which had been the first of this wheel arrangement in the USA, always retained their 69-inch drivers and became class P-69 when the class scheme was later introduced.

The first 39 were built by Schenectady and Brooks in 1910 and 1912, which had drivers of 69 inches, as stated before. They were numbered 6401 to 6439 and originally had a saturated boiler. They were already rebuilt in 1916, where they received a superheater and drivers with a diameter of 73 inches. Their cylinder diameter was increased by two inches, while the stroke was increased by one inch. Later they received 65 square feet of thermic syphons, what increased their direct heating surface to 271 square feet. Some also had Timken roller bearings and an Elasco feed water heater.

The first five locomotives which had 73-inch drivers from the start were built by Schenectady in 1921. They also had a superheater from the start, but still a grate area of only 49.5 square feet like the first ones. In 1924 and 1925, the same ALCO works built 29 which had a grate of 67.8 square feet and were fired with oil. After the Missouri Pacific had bought the International-Great Northern and the St. Louis, Brownsville & Mexico, Brooks built five and six locomotives for these two railroads which were nearly identical to the last batch. All these batches were retired between 1948 and 1954.